Phone keeps losing wifi?

For some reason my phone keeps forgetting the wifi at home.
That it doesn't stay connected all the time is fair enough, has to save battery, but when I put my phone back on it doesn't reconnect to my router, instead I have to put in the password again every time (long and annoying).
Any idea whats wrong here?
I'm really paranoid since windows phone seems very graphics intensive and I don't have unlimited internet.